2 Answers
2

Luke Skywalker has vanished. In his absence, the sinister FIRST ORDER has risen from the ashes of the Empire and will not rest until Skywalker, the last Jedi, has been destroyed.

With the support of the REPUBLIC, General Leia Organa leads a brave RESISTANCE. She is desperate to find her brother Luke and gain his help in restoring peace and justice to the galaxy.

Yes, the Empire has gone but the forces of oppression have a new face now...The First Order..

In the decades that followed the Empire's defeat, the First Order gradually built its strength through the secret mobilization of new fleets and armies in violation of the Galactic Concordance and Republic law. Despite further acts of defiance to the treaty, the Galactic Senate did not regard the First Order as a serious threat to the Republic. However, the Senate's inaction motivated the Rebel veteran Princess Leia Organa to found the Resistance.

Quite a bit of good information on the founding of both the First Order and the Resistance can also be found in the novel "Star Wars: Bloodline", and the "Aftermath" novel trilogy. If you're interested, here are a couple of links: Star Wars: Bloodline on Amazon and Star Wars: Aftermath (Book 1) on Amazon. All of these books are also available as audiobooks if you prefer.
– G_Hosa_PhatAug 2 '17 at 21:26

There is also the Journey to the Last Jedi Leia novel and Shattered Empire comic that also shed some light on this in between era. The Mandalorian and Resistance are surely going to add to this too!
– Darth LockeNov 17 '18 at 18:22

The Resistance was resisting the New Republic. The New Republic became as lazy as the old one, and didn't take the First Order seriously. Leia decided to fight the First Order, by founding a resistance movement.

Hello and Welcome! I have read that this is the situation PRIOR to TFA too and I think it adds to the other answer, but it would help if could edit your answer and then provide a source to better substantiate it.
– Darth LockeNov 17 '18 at 18:19